High frequency triode-type field emission device and process for manufacturing the same

ABSTRACT

Disclosed herein is a triode-type field emission device, in particular for high frequency applications, having a cathode electrode, an anode electrode spaced from the cathode electrode, a control gate electrode arranged between the anode electrode and the cathode electrode, and at least a field-emitting tip; the cathode, control gate and anode electrodes overlapping in a triode area at the field-emitting tip and being operable to cooperate with the field-emitting tip for generation of an electron beam in the triode area. The cathode, control gate and anode electrodes do not overlap outside the triode area, and have a main direction of extension along a respective line; each of these respective lines being inclined at a non-zero angle with respect to each one of the others.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to a micro/nanometrical devicebelonging to the family of semiconductor vacuum tubes for high frequencyapplications, and more particularly to an innovative high frequencytriode-type field emission device, and to a process for manufacturingthe same.

BACKGROUND ART

As is known, technology and applications in the THz frequency range havetraditionally been restricted to the field of molecular astronomy andchemical spectroscopy. However, recent advances in THz detectors andsources have opened the field to new applications, including homelandsecurity, measurement systems (network analysis, imaging), biologicaland medical applications (cell characterization, thermal and spectralmapping), material characterization (near-field probing, food industryquality control, pharmaceutical quality control).

Although commercial uses for THz sensors and sources are growing, thisgrowth is somehow limited by the difficulty of providing reliable THzsources, for which traditional semiconductor technology, due to poorelectron mobility, has proven not satisfactory.

Use of vacuum electronics instead of semiconductor technology allows toexploit the property of electrons of reaching higher speeds in vacuumthan in a semiconductor material, and thus to reach higher operatingfrequencies (nominally from GHz to THz). The general working principleof vacuum electronic devices is based on the interaction between an RFsignal and a generated electron beam; the RF signal imposes a velocitymodulation to the electrons of the electron beam permitting an energytransfer from the electron beam to the RF signal.

Conventional old-generation vacuum tubes included thermionic cathodesfor generating the electron beam, operating at very high temperature(800° C.-1200° C.), and suffered from many limitations, among which:high electric power requirements, high heating-up time, instabilityproblems and limited miniaturization.

The above limitations have been overcome with the introduction of vacuumdevices with a FEA (Field Emission Array) cathode, that has led tosignificant advantages, in particular for THz frequency amplification,allowing to work at room temperature, and to achieve size reduction downto the micro- and nanometric dimensions. A FEA structure for RF sourceswas first proposed by Charles Spindt (C. A. Spindt et al., Physicalproperties of thin-film field emission cathodes with molybdenum cones,Journal of Applied Physics, vol. 47, December 1976, pages 5248-5263),and is usually referred to as the Spindt cathode (or cold cathode, dueto the low operating temperature). In particular, Spindt cathode devicesconsist of micromachined metal field emitter cones or tips formed on aconductive substrate, and in ohmic contact therewith. Each emitter hasits own concentric aperture in an accelerating field between an anodeand a cathode electrodes; a gate electrode, also known as control grid,is isolated from the anode and cathode electrodes and the emitters by asilicon dioxide layer. With individual tips capable of yielding severaltens of microamperes, large arrays can theoretically produce largeemission current densities.

Performance of Spindt cathode devices are limited by damaging of theemitting tips due to material wear, and for this reason many effortshave been spent worldwide in searching innovative materials for theirproduction.

In particular, the Spindt structure was much improved by using CarbonNanotubes (CNTs) as cold cathode emitters (see for example S. Iijima,Helical microtubules of graphitic carbon, Nature, 1991, volume 354,pages 56-58, or W. Heer, A. Chatelain, D. Ugarte, A carbon nanotubefield-emission electron source, Science, 1995, volume 270, number 5239,pages 1179-1180). Carbon nanotubes are perfectly graphitized,cylindrical tubes that can be produced with diameters ranging from about2 to 100 nm, and lengths of several microns using various manufacturingprocesses. In particular, CNTs may be rated among the best emitters innature (see for example J. M. Bonard, J.-P. Salvetat, T. Stockli, L.Forrò, A. Chatelain, Field emission from carbon nanotubes: perspectivesfor applications and clues to the emission mechanism, Applied Physics A,1999, volume 69, pages 245-254), and therefore are ideal field emittersin a Spindt-type device; many studies have already acknowledged theirfield emission properties (see for example S. Orlanducci, V. Sessa, M.L. Terranova, M. Rossi, D. Manna, Chinese Physics Letters, 2003, volume367, pages 109-114).

In this regard, FIG. 1 shows a schematic sectional view of a knownSpindt-type cold cathode triode device 1, using CNTs as field emitters.The triode device 1 comprises a cathode structure 2; an anode electrode3 spaced from the cathode structure 2 by means of lateral spacers 4; anda control gate 5 integrated in the cathode structure 2. The cathodestructure 2 with the integrated control gate 5, and the anode electrode3, are formed separately and then bonded together with the interpositionof the lateral spacers 4. The anode electrode 3 is made up of a firstconductive substrate functioning as the anode of the triode device,while the cathode structure 2 is a multilayer structure including: asecond conductive substrate 7; an insulating layer 8 arranged betweenthe second conductive substrate 7 and the control gate 5; a recess 9formed to penetrate the control gate 5 and the insulating layer 8 so asto expose a surface of the second conductive substrate 7; andSpindt-type emitting tips 10 (only one of which is shown in FIG. 1, forsimplicity of illustration), in particular CNTs, formed in the recess 9in ohmic contact with the second conductive substrate 7, and functioningas the cathode of the triode device.

During operation, biasing of the control gate 5 allows controlling theflow of electrons generated by the cathode structure 2 towards the anodeelectrode 3, at the area corresponding to and surrounding the recess 9;the current thus generated is collected by the portion of the anodeelectrode 3 that is placed over the control gate 5.

In the triode device 1, a triode (or active) area can thus be defined(denoted with 1 a in FIG. 1), including the region at, and closelysurrounding, the emitting tips 10 and recess 9, in which electrons aregenerated and collected; and a triode biasing area 1 b, as the regionoutside and external to the triode area 1 a, through which biasingsignals are conveyed to the same triode area.

OBJECT AND SUMMARY OF THE INVENTION

The Applicant has noticed that the topographic configuration of knownSpindt-type vacuum tube triode devices suffers from an importantlimitation, due to the large value of parasitic capacitances existingbetween the control gate and the cathode and anode electrodes. Thisparasitic capacitance heavily limits the operating frequency that thistype of device can reach, reducing the cut-off frequency, and making THzapplications, even for micron scaled structures, substantiallyunfeasible.

In particular, known realization of the cold cathode devices envisagesthe presence of an extended control gate, which overlaps the conductivecathode substrate, thus forming two plates of a parasitic capacitor(denoted with C_(GC) and shown schematically in FIG. 1). In detail, andassuming the control gate and cathode substrate to be modeled as twoflat and parallel plates, the value of this parasitic gate-cathodecapacitance C_(GC) is given by C=e₀e_(r)(A/d), wherein e₀ is the vacuumpermittivity, e_(r) is the relative permittivity of the insulatingmaterial between the cathode and the control gate, A is the area ofoverlap, and d is the distance between the cathode and the control gate.The parasitic gate-cathode capacitance C_(GC) is also much larger thanthe capacitance between the control gate and the emitting tip (denotedwith C_(GT) in FIG. 1).

Moreover, the overlap between the anode electrode and the control gategenerates a further parasitic capacitance, the gate-anode capacitance(denoted with C_(GA) and shown schematically in FIG. 1), that adds up tothe overall parasitic capacitance, determining a further degradation ofthe cut-off frequency of the device.

From the foregoing, it is evident that the operating frequency of thistype of device is heavily dependent on, and strongly limited by, itstopographic characteristics.

The main objective of the present invention is thus to provide aninnovative topographical configuration for cold cathode vacuum tubes andan innovative manufacturing process, for the aforementioned drawback tobe at least in part overcome.

This objective is achieved by the present invention in that it relatesto a high frequency triode-type field emission device, and to a relatedmanufacturing process, as defined in the appended claims.

The present invention achieves the aforementioned objective by varyingthe typical topography of a triode-type field emission device, andparticularly by limiting the area of overlap between the cathode andanode electrodes and the control gate, thus reducing the value of theoverall parasitic capacitance formed therebetween; the overlap betweenthe different conductive surfaces is indeed limited to a triode area ofthe field emission device.

In detail, the control gate, anode and cathode electrodes are composedof a respective strip-shaped conduction line leading to a respectiveterminal; the various electrodes overlap only at the triode area (inparticular with the terminals thereof, allowing generation andcollection of the electron beam), while the various conduction lines areso arranged as not to overlap each other outside the same triode area.In more detail, the conduction lines, conducting electrical signalsto/from the respective terminals, are inclined, one with respect to eachof the other, at a non-zero angle, in particular at an angle of 60° (or120°, if the complementary angle between any of the two lines isconsidered).

The advantages of the proposed structure are particularly significant incathode array structures where contributions of all parasiticcapacitances add up; in particular, the possibility of realizing largearrays of cold cathode devices without suffering for frequencylimitation due to parasitic capacitances is one of the key issues ofthis structure.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, preferredembodiments, which are intended purely by way of example and are not tobe construed as limiting, will now be described with reference to theattached drawings (all not drawn to scale), wherein:

FIG. 1 shows a schematic cross-sectional view of a known Spindt-typecold cathode triode with a CNT as field emitter, and with parasiticcapacitances highlighted;

FIG. 2 is a schematic top view of a high frequency triode-type fieldemission device according to the present invention;

FIG. 3 is a schematic perspective exploded view of the high frequencytriode-type field emission device of FIG. 2;

FIG. 4 is a cross sectional view of the high frequency triode-type fieldemission device according to a first embodiment of the presentinvention;

FIGS. 5 a-5 f are perspective views of a semiconductor wafer duringsuccessive steps of a process for manufacturing a cathode structure ofthe high frequency triode-type field emission device, according to thefirst embodiment of the present invention;

FIG. 6 is a cross sectional view of a high frequency triode-type fieldemission device according to a second embodiment of the presentinvention;

FIG. 7 is a variant of the high frequency triode-type field emissiondevice of FIG. 6; and

FIG. 8 is a schematic top view of an array of high frequency triode-typefield emission devices according to a further embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The following discussion is presented to enable a person skilled in theart to make and use the invention. Various modifications to theembodiments described will be readily apparent to those skilled in theart, and the generic principles herein may be applied to otherembodiments and applications without departing from the spirit and scopeof the present invention. Thus, the present invention is not intended tobe limited to the embodiments shown, but is to be accorded the widestscope consistent with the principles and features disclosed herein anddefined in the attached claims. FIGS. 2 and 3 show respectively aschematic top view and a perspective exploded view of a high-frequencytriode-type field emission device 11 according to the present inventionand defined as having a “crossbar structure”, while FIG. 4 shows a crosssectional view of the high frequency triode-type field emission device11, in accordance with a first embodiment of the present invention.

In detail, according to the first embodiment of the present invention,the high-frequency triode-type field emission device 11 comprises: amultilayered structure integrating a cathode electrode 12 and a controlgate (or control grid) electrode 13; and an anode electrode 14, that isbonded to this multilayered structure, using vacuum bonding techniques,with lateral spacer 15 in order to maintain electrical isolationtherebetween.

In more detail, the cathode electrode 12 is arranged over a substrate,in particular a multilayer substrate 16 including: a thick insulatinglayer 16 c, that acts as a support for the whole structure; a conductinglayer 16 a, made of silicon or other semiconductor or conductingmaterials and acting as a ground plane for the device; and an overlyinginsulating layer 16 b, made e.g. of silicon oxide. The cathode electrode12 includes a cathode conduction line 12 a and a cathode terminal 12 b,the latter having a full disc shape. The cathode conduction line 12 ahas a strip-like shape with a main extension direction along a firstdirection x, leads to the cathode terminal 12 b, and crosses itextending from opposite portions thereof along the first direction x;the cathode conduction line 12 a is centered with respect to the cathodeterminal 12 b.

An insulating region 17, having the shape of an annulus, is arranged onthe multilayer substrate 16 and the cathode electrode 12, and defines afirst recess 18, formed therethrough so as to expose a top surface ofthe cathode terminal 12 b. Spindt-type emitting tips 19 (only one ofwhich is shown in FIGS. 2-4, for simplicity of illustration), inparticular CNTs, are arranged on the exposed top surface of the cathodeelectrode 12 b within the first recess 18.

The control gate electrode 13 is arranged over, and partially overlapsthe cathode electrode 12, in particular it overlaps partially thecathode conduction lines 12 a at a triode area 11 a of the device(which, as previously, is defined as the area at, and closelysurrounding, the emitting tips 19 and first recess 18, in whichelectrons are generated and collected). The control gate electrode 13includes a gate conduction line 13 a and a gate terminal 13 b, thelatter having a ring or annulus shape with an inner radius, that is e.g.equal to the radius of the cathode terminal 12 b. The gate conductionline 13 a has a strip-like shape with a main extension direction along asecond direction y, and leads to the gate terminal 13 b, extending fromopposite portions thereof along the second direction y, without crossingit; the gate conduction line 13 a is centered with respect to the gateterminal 13 b. In particular, the first and second directions x, ydefine skew lines lying on parallel planes, and the second direction yis oriented by a non zero angle, in particular by an angle of 120° (or60°, if the complementary angle is considered) with respect to the firstdirection x (the angle between the two lines being defined as either ofthe angles between any two lines parallel to them and passing through asame point in space).

The anode electrode 14 is arranged over the cathode electrode 12 and thecontrol gate electrode 13, and partially overlaps them, in particular atthe triode area 11 a. The anode electrode 14 is formed on an insulatingsubstrate 20 that is bonded to the multilayered structure integratingthe cathode and control gate electrodes, with the interposition of thelateral spacer 15. In particular, the lateral spacer 15 has here anannulus shape and internally defines a second recess 21, that is equalto the first recess 18, and opens to the inside aperture of the gateterminal 13 b and the same first recess 18, allowing flow of thegenerated electrodes towards the anode electrode 14.

In greater detail, the anode electrode 14 includes an anode conductionline 14 a and an anode terminal 14 b, the latter having a full discshape with a radius equal to the radius of the cathode terminal 12 b.The anode conduction line 14 a has a strip-like shape with a mainextension direction along a third direction z, and extends along thethird direction z from opposite portions of the anode terminal 14 b,being centered thereto. In particular, the second and third directionsy, z are skew lines lying on parallel planes and the third direction zis oriented by a non zero angle, in particular by an angle of 120° (or60°, if the complementary angle is again considered) with respect to thesecond direction y. Consequently, each of the first, second and thirddirections x, y, z is oriented by an angle of 60° (120°) with respect toeach of the other ones.

From the foregoing description, it follows that overlapping between thedifferent conductive regions of the triode device, i.e. the cathode,control gate and anode electrodes 12, 13, 14, is limited to the triodearea 11 a thereof, at which electrons are generated and directed fromthe cathode terminal 12 b (and the emitting tips 19) to the anodeterminal 14 b. In particular, due to the structure spatial orientation,this overlap is limited to the cathode and anode terminals 12 b, 14 b(which fully overlap), and to a partial overlap between the gateterminal 13 b and the cathode and anode conduction lines 12 a, 14 a.Advantageously; the cathode, gate and anode conduction lines 12 a, 13 a,14 a do not overlap each other.

FIGS. 5 a-5 f (where same reference numerals designate same elements asones described before) show successive steps of the process formanufacturing the multilayered structure integrating the cathode andcontrol gate electrodes of the high-frequency triode-type field emissiondevice 11, according to the first embodiment of the present invention.

In detail, FIG. 5 a, in an initial step of the process, a multilayeredsubstrate 16 is provided, having an insulating layer 16 b, e.g. a 4-μmoxide layer, formed by deposition or oxidation on a conducting layer 16a, made of silicon and having a thickness ranging from 2 to 10 μm (theconducting layer 16 a acting as the ground plane of the device); theconducting layer 16 a is realized on a thick insulating layer 16 c (madeof silicon dioxide or quartz).

Next, FIG. 5 b, a first metal layer is formed, e.g. by deposition, onthe insulating layer 16 b; a photoresist pattern (not shown) is definedon the first metal layer, and the same layer is etched to define thecathode electrode 12, having a strip-shaped cathode conduction line 12 aand a disc-shaped cathode terminal 12 b, coupled to the conduction line.

Using known techniques, such as for example e-beam lithography, aphotoresist pattern (not shown) is aligned on the multilayered substrate16, and a catalyst film (Fe or Ni) is deposited, e.g. by sputtering, andthen lifted-off so as to leave only a catalyst region 24 (FIG. 5 c) onthe cathode terminal 12 b, in particular at a center portion thereof.The thickness of the catalyst film is in the range of tens of nanometers(e.g. 5-50 nm).

Using a further alignment, an insulating layer is deposited e.g. bysputtering, and then lifted-off, for the formation, FIG. 5 d, of aninsulating region 17, having the shape of an annulus surrounding thecatalyst region 24. The insulating region 17 is designed to insulate thecathode conduction line 12 a from the control gate terminal. Theinsulating layer is made of silicon oxide with a thickness in the rangeof microns.

Again using a proper alignment, a second metal layer (not shown), forexample of niobium, having a thickness of about 100 nm, is deposited andthen lifted-off, so as to define the control gate electrode 13 (FIG. 5e). In particular, the control gate electrode 13 comprises a gateconduction line 13 a, inclined at a non-zero angle with respect to thecathode conduction line 12 a, and a gate terminal 13 b, having anannulus shape with an inner opening facing the catalyst region 24. Then,an anodization process is carried out on the gate electrode 13, in orderto reduce the current losses and to protect the same gate electrodeduring a subsequent CNT synthesis process.

Next, FIG. 5 f, the structure is submitted to CNTs synthesis in order toobtain (in a per se known manner) Spindt-type emitting tips 19; inparticular, CNTs as field emitters are formed on the catalyst region 24.

The multilayered structure formed as described above and the anodeelectrode 14 are then aligned (taking into account the desired mutualorientation) and bonded together with the interposition of the lateralspacer 15, creating vacuum therebetween. In particular, the anodeelectrode 14 is first formed on the insulating substrate 20 (which ismade e.g. of glass or silicon oxide), using common patterningtechniques, and then the insulating substrate 20 is bonded to themultilayered structure using standard wafer-to-wafer vacuum bondingtechniques, such as anodic bonding, glass frit bonding, eutecticbonding, solder bonding, reactive bonding or fusion bonding.

Given that a high quality vacuum is advantageous for ensuring reliableoperation of the high-frequency triode-type field emission device 11, avariant of the described process (not shown in the Figures) may envisagethe formation of a region containing a suitable reactive material suchas Ba, Al, Ti, Zr, V, Fe, commonly known as a getter region. The getterregion may allow, when appropriately activated, molecules desorbedduring the bonding process to be captured. For a detailed description ofthe use of getter material to improve vacuum bonding, reference may bemade to Douglas R. Sparks, S. Massoud-Ansari, and Nader Najafi,Chip-Level Vacuum Packaging of Micromachines Using NanoGetters, IEEEtransactions on advanced packaging, volume 26, number 3, August 2003,pages 277-282, and Yufeng Jin, Zhenfeng Wang, Lei Zhao, Peck Cheng Lim,Jun Wei and Chee Khuen Wong, Zr/V/Fe thick film for vacuum packaging ofMEMS, Journal of Micromechanics and Microengineering, volume 14, 2004,pages 687-692. In a way not shown, this getter region may for example beformed close to the anode electrode 14 inside the second recess 21 (thelateral spacer 15 being arranged so as to leave space for the formationof the getter region).

According to a second embodiment of the high-frequency triode-type fieldemission device 11, the control gate electrode 13 is integrated with theanode electrode 14, forming a multilayered structure therewith, insteadof being integrated with the cathode electrode 12. This differentstructure has some specific advantages, as discussed in detail inco-pending patent application PCT/IT2006/000883 filed in the name of thesame Applicant on 29 Dec. 2006, and in particular may prevent shortcircuits occurring between the control gate electrode 13 and theemitting tips 19, and further reduce the value of parasiticcapacitances. The mutual spatial arrangement of the cathode, controlgate and anode electrodes 12, 13, 14 does not change, so that mutualoverlap is still limited to the triode area 11 a, as previouslydiscussed in detail. Since the second embodiment can be realized withsimple modifications of the manufacturing process described for thefirst embodiment, the related manufacturing process will not bedescribed again.

In detail, FIG. 6, the anode electrode 14 is in this case formed on themultilayer substrate 16, again including the thick insulating layer 16c, the conducting layer 16 a, acting as a ground plane for the device,and the overlying insulating layer 16 b in contact with the anodeelectrode 14. The insulating region 17 is arranged on the multilayersubstrate 16 and the anode electrode 14, and defines the first recess18, exposing a top surface of the anode terminal 14 b. The control gateelectrode 13 is arranged on the insulating region 17, with the inneropening of the gate terminal 13 b open to the first recess 18.

The cathode electrode 12 is patterned on the insulating substrate 20,and the emitting tips 19 are formed on the exposed top surface of thecathode terminal 12 b. The cathode electrode 12 and insulating substrate20 are then bonded to the multilayer structure integrating the controlgate and anode electrodes 13, 14, with the lateral spacers 15maintaining electrical isolation therebetween.

A possible variant of this second embodiment, FIG. 7, may provide forthe ground plane (conducting layer 16 a) to be coupled to the insulatingsubstrate 20; the cathode electrode 12 is in this case patterned on themultilayer structure made by the insulating substrate 20 formed on theconducting layer 16 a. The anode electrode 14, which is integrated withthe control gate electrode 13, is instead formed on the insulating layer16 b.

FIG. 8 shows a further embodiment of the present invention, envisagingthe formation of an array 25 of a large number of high-frequencytriode-type field emission devices 11, having the previously described“cross-bar structure”.

In detail, the high-frequency triode-type field emission devices 11 ofthe array 25 are aligned along the first, second and third direction x,y, z. Each of the high-frequency triode-type field emission devices 11in the array 25 shares its cathode, gate and anode conduction lines 12a, 13 a, 14 a, with other devices, with which it is aligned along thefirst, second and third direction x, y, z, respectively. As a result,the devices aligned in the first, second or third direction share acommon conduction line, and in particular the cathode, gate or anodeconduction line 12 a, 13 a, 14 a directed along that direction; thehigh-frequency triode-type field emission devices 11 are thus arrangedin an hexagonal lattice, providing for a regular, rational and compactarea occupation.

The advantages of the triode-type field emission device according to thepresent invention are clear from the foregoing.

In particular, the envisaged cross-bar structure arrangement allows tostrongly reduce the parasitic capacitance effects, and to really extendthe operating frequency band of the device in the THz frequency range.This is mainly due to the overlap among the different metal surfaces(gate, cathode and anode electrodes) being limited to the triode area ofthe device, while outside the triode area no overlap is provided betweenthese surfaces (and in particular between the various conduction lines).Thus, the value of the overall parasitic capacitance is heavily reduced.

A simple estimation of the maximum overlapping area to achieve a cut-offfrequency of at least 1 THz is possible by considering commonly usedexpressions. In particular, considering a distance of 2 g_(m) betweenthe cathode and gate terminals 12 b, 13 b, it is possible to estimatethat a maximum overlapping area of 20.000 nm² is requested to yield acut-off frequency of 0.1 THz. An overlapping area with this value caneasily be achieved by using an anodic and cathode circular area with aradius in the range of 0.5 μm, the cathode, gate and anode conductionlines 12 a, 13 a, 14 a having a section of e.g. 0.1 μm.

With this arrangement, the estimated parasitic capacitance is in therange of 10⁻¹⁸ F, therefore taking into account a value oftransconductance g_(m) in the range of 0.1-50 μS and a DC gain in therange of 1-500 (see for example W. P. Kang, Y. M. Wong, J. L. Davidson,D. V. Kerns, B. K. Choi, J. H. Huang and K. F. Galloway, Carbonnanotubes vacuum field emission differential amplifier integratedcircuits, Electronics Letters Vol. 42 No. 4, 2006 and Y. M. Wong, W. P.Kang, J. L. Davidson, J. H. Huang, Carbon nanotubes field emissionintegrated triode amplifier array, Diamond & Related Materials, vol. 15,p. 1990-1993, 2006) the cut-off frequency is in the range of THz.

Moreover, the described cross-bar structure, due to the reducedparasitic capacitance, is well suited for the integration of largearrays of field emitter devices in the THz frequency range. Inparticular, the chosen orientation for the conduction lines of thecathode, gate and anode electrodes 12, 13, 14, and in particular theinclination angle of 120°, allows to achieve a very limited overlaparea, together with a rational integration of the array and a reducedarea occupation, and it is accordingly particularly advantageous.

The realization of the proposed structure is well suited for CNT Spindtcold cathodes, since CNTs can be grown in well defined position by theuse of a suitably patterned catalyst.

Furthermore, integration of the anode and control gate electrodes in asame structure (as shown in FIGS. 6 and 7) may prove particularlyadvantageous, in order to further improve the electrical performances ofthe triode-type field emission device.

Finally, numerous modifications and variants can be made to thetriode-type field emission device according to the present invention,all falling within the scope of the invention, as defined in theappended claims.

In particular, an initial step of the manufacturing process may envisagethe provision of a SOI (Silicon On Insulator) multilayered substrate; inthis case, the cathode electrode 12 (according to the first embodiment),or anode electrode 14 (according to second embodiment), may be formed bypatterning of the silicon active layer of the SOI substrate, withouthaving to deposit and etch an additional metal layer. SOI substrateshave indeed already demonstrated to be suitable for the synthesis ofcarbon nanotubes.

Moreover, the internal vertical sides of the control gate electrode 13could be spaced out from the internal vertical sides of the insulatingregion 17 (and the inner radius of the control gate electrode 13 thus behigher than the radius of the cathode and anode terminals 12 b, 14 b),so as to be covered by the lateral spacers 15 during the bondingprocess; this solution may allow a reduction of the leakage currents.

A variant of FIG. 4 could also be envisaged, corresponding to that ofFIG. 7, having the conductive layer 16 a (the ground plane) coupled tothe insulating substrate 20 and not to the insulating layer 16 b.

Additionally, it may readily be appreciated that the thickness of thevarious layers of the device and the various steps of the manufacturingprocess are only indicative and may be varied according to specificneeds. In particular, for sake of simplicity, the description of themanufacturing process has made reference to manufacturing of a singlecathode structure; however, the manufacture of an array of cathodestructures simply requires the use of modified lithographical masks inwhich a same base structure is repeated.

The invention claimed is:
 1. A triode-type field emission device, inparticular for high frequency applications, comprising: a multilayeredstructure integrating: a cathode electrode, an anode electrode spacedfrom the cathode electrode, a control gate electrode arranged betweensaid anode electrode and said cathode electrode, and at least afield-emitting tip; wherein i) said cathode electrode, control gateelectrode, and anode electrode are formed to overlap within a triodearea at said field-emitting tip and to cooperate with saidfield-emitting tip to generate an electron beam in said triode area, andii) at most two of said cathode electrode, control gate electrode, andanode electrode overlap at any point outside said triode area.
 2. Thedevice according to claim 1, wherein said multilayered structure furtherincludes a substrate comprising an electrically conductive layeroperable as a ground plane for said device, whereby said electron beamis substantially orthogonal to said electrically conductive layer. 3.The device according to claim 2, wherein said multilayered structure isa stacked structure.
 4. The device according to claim 1, wherein each ofsaid cathode electrode, control gate electrode, and anode electrode hasa main direction of extension along a respective line; each of saidrespective lines being inclined at a non-zero angle with respect to eachone of the others.
 5. The device according to claim 4, wherein saidangle is about 60°.
 6. The device according to claim 1, wherein each ofsaid cathode electrode, control gate electrode, and anode electrodeincludes a respective terminal at said triode area, and a respectiveconduction line extending from said respective terminal to a biasingarea outside said triode area, and operable to conduct electricalsignals for said respective terminal; the conduction lines of saidcathode electrode, control gate electrode, and anode electrode beingmutually arranged so as not to overlap.
 7. The device according to claim6, wherein said conduction lines of each of said cathode electrode,control gate electrode, and anode electrode extends along a respectiveline; each of said respective lines being inclined at a non-zero anglewith respect to each one of the others.
 8. The device according to claim7, wherein said angle is about 60°.
 9. The device according to claim 6,wherein the terminals of said cathode electrode and said anode electrodeoverlap at said triode area, and the terminal of said control gateelectrode partially overlaps the conduction lines of said cathodeelectrode and said anode electrode at said triode area.
 10. The deviceaccording to claim 9, wherein said conduction lines of said cathodeelectrode, control gate electrode, and anode electrode have a strip-likeshape, are connected to said respective terminal, and extend along arespective line from opposite portions of said respective terminal. 11.The device according to claim 9, wherein the terminal of said cathodeelectrode has a disc shape, and is surmounted by said field-emitting tipand is in ohmic contact therewith; the terminal of said control gateelectrode has an annulus shape defining a recess opening towards saidfield-emitting tip; and the terminal of said anode electrode has a discshape overlying said recess and field-emitting tip; an internal radiusof the terminal of said control gate electrode being no smaller than theradii of the terminals of said cathode electrode and anode electrode.12. The device according to claim 1, further comprising a cathodestructure including said cathode electrode and an anode structureincluding said anode electrode, said cathode and anode structures beingformed separately and bonded together with the interposition of spacers;wherein said control gate electrode is integrated in said anodestructure.
 13. An array comprising a plurality of triode-type fieldemission devices, in particular for high frequency applications, eachdevice comprising a multilayered structure integrating a cathodeelectrode, an anode electrode spaced from the cathode electrode, acontrol gate electrode arranged between said anode electrode and saidcathode electrode, and at least a field-emitting tip; wherein i) saidcathode electrode, control gate electrode, and anode electrode areformed to overlap at a triode area at said field-emitting tip and tocooperate with said field-emitting tip to generate an electron beam insaid triode area, and ii) at most two of said cathode electrode, controlgate electrode, and anode electrode overlap at any point outside saidtriode area.
 14. The array according to claim 13, wherein each of saidcathode electrode, control gate electrode, and anode electrodes has amain direction of extension along a respective line, each of saidrespective lines being inclined at a non-zero angle with respect to eachone of the others, and includes a respective conduction line arrangedalong said respective line; and wherein said triode-type field emissiondevices are aligned along said respective lines, the devices alignedalong a given line sharing a common conduction line, and in particularthe conduction line of said cathode electrode, control gate electrode,or anode electrode that is directed along said given line.
 15. The arrayaccording to claim 13, wherein said triode-type field emission devicesare arranged in an hexagonal lattice.
 16. A process for manufacturing atriode-type field emission device, in particular for high frequencyapplications, comprising forming a multilayered structure integrating acathode electrode, an anode electrode spaced from the cathode electrode,a control gate electrode arranged between said anode electrode and saidcathode electrode, and at least a field-emitting tip; wherein i) saidcathode electrode, control gate electrode, and anode electrode areformed to overlap at a triode area at said field-emitting tip and tocooperate with said field-emitting tip to generate an electron beam insaid triode area, and ii) at most two of said cathode electrode, controlgate electrode, and anode electrode overlap at any point outside saidtriode area.
 17. The process according to claim 16, wherein saidmultilayered structure further includes a substrate comprising anelectrically conductive layer operable as a ground plane for saiddevice, whereby said electron beam is substantially orthogonal to saidelectrically conductive layer.
 18. The process according to claim 17,wherein said multilayered structure is a stacked structure.
 19. Theprocess according to claim 16, wherein arranging includes arranging themain directions of extension of each of said cathode electrode, controlgate electrode, and anode electrode along a respective line; each ofsaid respective lines being inclined at a non-zero angle with respect toeach one of the others.
 20. The process according to claim 19, whereinsaid angle is about 60°.
 21. The process according to claim 16, whereinforming said cathode electrode, control gate electrode, and anodeelectrode includes forming a respective terminal thereof at said triodearea, and a respective conduction line thereof extending from saidrespective terminal to a biasing area outside said triode area, saidrespective conduction line operable to conduct electrical signals forsaid respective terminal; and wherein arranging includes mutuallyarranging the conduction lines of said cathode electrode, control gateelectrode, and anode electrode so as not to overlap.
 22. The processaccording to claim 21, wherein mutually arranging includes positioningsaid conduction lines of each of said cathode electrode, control gateelectrode, and anode electrode along a respective line; each of saidrespective lines being inclined at a non-zero angle with respect to eachone of the others.
 23. The process according to claim 22, wherein saidangle is about 60°.
 24. The process according to claim 21, whereinarranging includes arranging said terminals of said cathode electrodeand said anode electrode so as to overlap at said triode area, and theterminal of said control gate electrode so as to partially overlap theconduction lines of said cathode electrode and said anode electrode atsaid triode area.
 25. The process according to claim 16, furthercomprising forming separately a cathode structure and an anode structureon a respective insulating substrate, said step of forming a cathodestructure including forming said cathode electrode and said step offorming said anode structure including forming said anode electrode; andthen bonding together said cathode and anode structures with theinterposition of spacers; wherein forming said control gate electrodeincludes integrating said control gate electrode in said anodestructure.